Method and device for homogenizing glass melt

ABSTRACT

The invention relates to a device for homogenizing a glass melt in a melt receptacle, wherein at least one stirring device is disposed in a melt receptacle, which comprises a stirrer shaft and a plurality of stirrer blades, and wherein a gap ( 16 ) is formed between a wall region of the melt receptacle and the stirrer blades. According to the invention, the respective stirring device causes an axial feed action in an inner stirring region between the stirrer shaft and the stirrer blades in order to feed the melt in the stirring region along the stirrer shaft. A melt flow brought about by the axial feed action seals the gap against direct passage of the melt. According to the invention, a very high gap width can be achieved, thus preventing the abrasion of materials in the region of the marginal gap. This also reduces the complexity required for adjusting the device. According to the invention, a high level of homogenization can be achieved regardless of the entry point of the inhomogeneities.

This is a divisional of U.S. patent application Ser. No. 11/957,727,which was filed on Dec. 17, 2007, under 35 U.S.C. 120. The aforesaidU.S. patent application claims the benefit of priority based on GermanPatent Application, DE 10 2006 060 9723, filed on Dec. 20, 2006 inGermany.

FIELD OF THE INVENTION

The present invention relates to the homogenizing of a glass melt, andparticularly to the homogenizing of a glass melt used for the productionof a glass or glass ceramic product having high quality and a lowinclusions and/or imperfection density, such as display glass.

BACKGROUND OF THE INVENTION

The objective of homogenizing a glass melt is to reduce spatial andtemporal variations in the chemical composition of the glass melt, inaccordance with the product requirements. Chemical inhomogeneities canresult in inhomogeneities in the refractive index, which may impairoptical depiction, for example, and in inhomogeneities in the viscosity,which may result, for example, in uncontrolled geometrical variationsduring hot finishing processes or hot processing. To this end, adifferentiation is made between macro-inhomogeneities, which is to say avariation in chemical composition on comparatively large spatial scalesof for example, a few centimeters having small spatial gradients, andmicro-inhomogeneities (also referred to as striations), which is to saya variation of the chemical composition on small spatial scales of, forexample, 0.1 to 2 nm having in part large spatial gradients. The goal ofthe homogenizing process is to eliminate macro-inhomogeneities andmicro-inhomogeneities to as great an extent as is possible so that, forexample, a smooth progression of the refractive index can be obtained.

Glass melts are characterized in that, in typically used stirringsystems, they have viscosities ranging between 1 and 200 Pa·s, whichresults in laminar flow of the glass melt (Reynolds number<1), and inthat the chemical coefficient of diffusion is generally less than 10⁻¹²m²/s, so that homogenization that can be achieved by way of diffusion isnegligibly small. Rather, homogenization of glass melts can generallyonly be achieved by considerably expanding, redistributing and choppinglocal inhomogeneities and/or striations. For this purpose stirringsystems are used, which comprise a melt receptacle for temporarilyreceiving the glass melt and at least one stirring device for stirringthe glass melt in the melt receptacle.

In order to achieve any suitable homogenization under the aboveconditions, particularly with high viscosities and small chemicalcoefficients of diffusion, the gap between the stirrer blades of thestirring device and the wall of the melt receptacle is conventionallykept to a minimum. An excessively narrow gap between the stirrer bladesand the melt receptacle wall, however, poses the risk of the stirrercoming into contact with the wall of the melt receptacle, withconsequent damage to the stirrer and/or the stirrer vessel. Here, itmust be remembered that the stirrer can only ever be adjusted when themelt receptacle is in a cooled state. Because thermally induceddeformation of the stirrer or of the stirring system are unavoidablewhen heating to operating temperatures, the adjustment of the componentsis often no longer correct at the operating temperatures. This canresult in an excessively narrow distance between the stirrer blades andthe melt receptacle wall, and thus in direct contact with the material,which ultimately results in destruction of the stirring system.

The relative marginal gap width, i.e. the ratio 0.5*(diameter of thestirring device or diameter of the melt receptacle minus diameter of thestirrer)/(diameter of the stirring device or of the melt receptacle), istypically less than approximately 5%, or even less than approximately1%, of the melt receptacle diameter or of the diameter of the stirringdevice. Due to the aforementioned thermal deformation of the componentswhen heating the device to the operating temperature, the width of thegap cannot be consistently maintained, so that large marginal gaps musttypically be specified. For this reason, only unsatisfactoryhomogenization results are achieved with the state of the art,particularly for high-viscosity glass melts.

High shear stress between the stirrer blades and melt receptacle walldue to a narrow marginal gap can considerably impair the service life ofthe stirring system. In addition, there is the risk that, if thestirring gap is excessively narrow, bubbles adhering to the meltreceptacle wall may be sheared off and transferred into the product.High shear stresses can also bring about abrasion of the wall materialof the melt receptacle or stirrer vessel, resulting in micro-inclusionsin the glass or the glass ceramic, which are not desirable, particularlyin display glass products.

US 2003/0101750 A1 discloses a method and a device for homogenizing aglass melt for the production of display glass. At a predefined stirringefficiency, which is determined by the stirrer diameter, stirrer speedand marginal gap, a predefined shear rate is selected. The marginal gapis comparatively narrow and corresponds approximately to 6% to 9% of thefree diameter of the stirrer vessel.

Furthermore homogenization can also be achieved by the geometry of theactual stirrer blades. The inclination of the stirrer blades and hencethe feed action of the stirrer are preferably set such that the bladesoperate counter to the glass flow in the glass melt receptacle. To thisend, an axial feed action can be achieved by the angle of the stirrerblades, by the geometric shape of the stirrer blades and/or by a helicalarrangement of the stirrer blades on the stirrer shaft. For example, JP10265226 A discloses a configuration, wherein, the inner stirrer bladesfeed downward, while the outer stirrer blades feed upward so as toachieve improved homogenization. JP 63008226 A discloses that theinclination of the stirrer blades, and hence the feed action of thestirrer, can be adjusted so that the blades operate counter to the glassflow. In this way, dead space in the glass melt receptacle should beavoided.

For the reasons given above, according to the state of the art, thesmallest possible marginal gap is always desirable with a view toachieving the highest possible homogeneity.

U.S. Pat. No. 2,831,664 discloses a method and a device for homogenizinga glass melt, comprising a stirring device having a plurality of stirrerblades axially offset in relation to one another. The stirring device isdisposed in a cylindrical stirrer pot, which is provided with an inletfor the glass melt at an upper edge and an outlet for the glass melt atthe lower end. In a marginal gap between the inside wall of the stirrerpot and the stirrer blades the stirrer blades form a plurality ofregions having radial and at the same time vertical glass flow. Thedimensions of the stirring device produce a very narrow marginal gap,resulting in very high material stresses caused by the very high shearrates that are applied.

JP 2001-72426 A and the English abstract thereof disclose a device forhomogenizing a glass melt. The stirring device is disposed in acylindrical stirrer pot, which is provided with an inlet for the glassmelt at an upper end and an outlet for the glass melt at the lower end.The glass flows in the marginal gap between the inside wall of thestirrer pot and the stirrer blades and also in the stirrer circuit areflows flowing in the same direction in relation to the superimposedthroughput flow. This results in a comparatively poor homogenizationresult.

US 2002/0023464 A1 discloses a device for homogenizing a glass melt,comprising a centerline recirculation channel, specifically on theinside of the mixing shaft, or a separate external recirculationchannel. The glass melt consequently does not flow back in a marginalgap as defined by the present invention. A very narrow gap between theinside wall of the stirrer pot and the mixing blades is disclosed, whichproduces a very high mechanical load on the stirrer and the stirrervessel.

US 2003/0101750 A1 discloses a method, which is modified compared to theaforementioned U.S. Pat. No. 2,831,664, wherein the disadvantage of avery narrow marginal gap is mitigated in that the stirrer system isenlarged almost to scale in order to guarantee homogeneity withincreased mass throughput. This is achieved either by increasing therotational speed or by enlarging the stirrer volume. A rotational speedincrease, however, brings about an increased shear rate and thus ahigher precious metal exposure level, including the undesirablegeneration of precious metal particles in the stirrer vessel. Anenlarged stirrer volume is associated with higher material use andcosts.

Both solutions are mathematically defined with the help of anon-dimensional homogeneity number H, which defines the homogenizationpotential of the stirring device. It is apparent that, at a fixedhomogeneity number H and predefined throughput, the rotational speed ofthe stirring device is considered in a linear fashion and the size ofthe stirrer system is considered, in the case of geometric similarity,only with the reciprocal third root (cubic root). A desiredhomogenization level can thus be implemented much more easily with thehelp of a to-scale enlargement of the stirrer system than with arotational speed increase, particularly since a rising rotational speedincreases the shear forces and material stress or particle abrasion inthe marginal gap.

SUMMARY OF THE INVENTION

Despite various efforts in the state of the art, there is a continuedneed for methods and devices that enable even more efficienthomogenization of glass melts. In particular, according to the presentinvention a method and a device for homogenizing a glass melt are to beprovided, wherein high homogeneity can be achieved while applying lowstress on the components of the device, enabling easily and preciselyadjusting the device, and creating minimized abrasion or a low bubbleshear rate.

The present invention is thus based on a method for homogenizing a glassmelt in a melt receptacle that serves as a stirrer vessel, such as acylindrical vessel or a melt channel, wherein at least one stirringdevice is disposed in the melt receptacle, which comprises a stirrershaft and a plurality of stirrer blades carried by the stirrer shaft andprojecting therefrom, wherein a gap or stirring gap is formed between awall region of the melt receptacle and the stirrer blades.

According to the invention, the stirring device or the apparatus isconfigured such that in an inner stirring region of the stirring device,which is to say between the stirrer shaft and the stirrer blades, anaxial feed action is applied in order to feed the glass melt in theinner stirring region along the stirrer shaft. By suitably configuringthe stirring device and/or apparatus, further according to theinvention, the axial feed action is applied so that a melt flow broughtabout by the axial feed action seals the gap between the wall region ofthe melt receptacle and the stirrer blades against direct passage of theglass melt.

Surprisingly, it was found that the dynamic sealing of the marginal gapaccording to the invention enables excellent homogenization of glassmelts, particularly high-viscosity glass melts, despite considerablylarger marginal gap widths. Thus, according to the present invention,considerably larger marginal gap widths can be used than has beenconventionally possible. Due to the considerably larger marginal gapwidths, according to the invention, the stress on the components of thedevice can be significantly reduced. According to the invention,negligible material abrasion and a low bubble shear rate can, inparticular, be achieved, while the complexity required to adjust thecomponents of the device is kept low.

Thus, according to the invention, all glass inhomogeneities, regardlessof the point of entry into the stirrer system, reach the inner stirringregion between the stirrer shaft and the ends of the stirrer blades andare reduced there by means of expansion, chopping and spatialredistribution. With the method according to the present invention,comparatively large gap widths can be achieved between the stirrerblades and the inside wall of the melt receptacle. In this way,interferences caused by high shear rates, such as abrasion, corrosion orinclusions due to abrasion of the lining material of the melt receptacleand/or stirrer blade material, can be prevented. In order to achieve thesealing effect, it is not essential according to the invention for themelt flow brought about by the axial feed action of the respectivestirring device to actually move counter to the entering glass melt.Rather, it is sufficient that the gap be actively or dynamically sealedin the manner of a stopper made of glass melt, which is to say due to anaccumulation of glass melt material. Preferably, however, a flow ispresent in the gap, which is directed counter to the direction of theaxial feed action applied by the stirring device, so that the entireinflowing glass melt is entrained to the upper end of the stirringdevice by the glass melt ascending in the marginal gap. In any case,direct passage of the inflowing glass melt through the marginal gap tothe outlet of the stirrer vessel or melt receptacle is prevented.

According to a further embodiment, direct entry of the glass melt intothe inner stirring region is actively or dynamically prevented by one ormore stirrer blades. To this end, particularly, a reorientation of theentering glass flow can be brought about, for example toward an axialend of the inner stirring region, from where the entering glass melt isfed to an opposite axial end of the inner stirring region or of thestirrer shaft, where the flow actively or dynamically contributes tosealing the gap between the wall region of the melt receptacle and thestirrer blades.

According to a further embodiment, the stirrer blades of the stirringdevice extend across a portion of the cross-section of the inlet of themelt receptacle. Thus, a certain portion of the cross-section of themelt flow entering through the inlet is covered by the stirrer blades inorder to prevent direct entry of the inflowing glass melt into the innerstirring region. The inflowing glass melt, regardless of the point ofentry, is rather diverted to the upper end of the stirring device and itis only there that the melt enters the inner stirring region. Thepercentage by which the cross-section of the inflowing glass melt iscovered by the stirrer blades can be at least 50%. Even betterhomogenization of the glass melt can be achieved according to a furtherembodiment, if the cross-section of the inflowing glass melt is coveredby more than two thirds by the stirrer blades. In contrast to the stateof the art, the stirrer blades thus protrude beyond the lower edge ofthe inlet.

According to a further embodiment, the axial feed action of the stirringdevice can be dimensioned such that, for example by a suitable increaseof the stirrer speed, multiple passages of the glass melt through theinner stirring region are brought about. In other words, the glass meltexiting the axial end of the inner stirring region flows through the gapbetween the wall region of the melt receptacle and the stirrer blades ina direction opposite to the axial feed direction in the inner stirringregion, thus achieving the active sealing of the aforementioned gapregion.

In order to achieve the axial feed action, optionally one or more of thefollowing parameters can be set as necessary: angles or inclination ofthe stirrer blades, geometric shape of the stirrer blades, helicalarrangement of the stirrer blades along the circumference of the stirrershaft, rotational speed of the stirrer, diameter of the stirring device,number of stirrer blades, feed action of the stirrer blades, and thelike.

The above parameters can notably be simulated and systematicallyestablished with the help of mathematical and/or physical simulations ofthe flow conditions in the glass melt receptacle such that, based onsuch a simulation, optimized stirring results can be achieved as afunction of the required specifications. For the physical simulation, inparticular, model systems having comparable or downscaled dimensions andviscosity levels can be used, and the homogenization can be visuallyobserved and optically evaluated by introducing color stripes in theinflowing viscous fluid.

According to a further embodiment, the melt receptacle that serves as astirrer vessel is a channel-shaped receptacle through which the glassmelt flows continuously. According to a further embodiment, the flowthrough the melt receptacle is discontinuous, which can be achieved, forexample, by intermittently replenishing the melt receptacle. The glassmelt then flows through the glass melt receptacle in a predefinedthroughput direction. According to a preferred further embodiment, theaxial feed action, which is brought about by the respective stirringdevice, occurs in the throughput direction of the glass melt.

According to a further embodiment, a plurality of virtual stirrervessels form under the axial feed action in the melt receptacle, thevessels being configured as described above, wherein the virtual stirrervessels are connected in series and the glass melt fed by an upstreamstirring device in the inner stirring region of a downstream stirringdevice is transferred to the axial end thereof such that direct entry ofthe glass melt, which is delivered by the upstream stirring device, intothe inner stirring region of the downstream stirring device is activelyprevented by one or more stirrer blades. In each of the virtual stirrervessels, the glass melt is homogenized as a function of the respectivelyselected parameters of the stirrer vessel, achieving an overallhomogenization level that is the nth power of the homogenization levelof an individual stirrer vessel. Such an embodiment is particularlysuited for a melt receptacle configured as a melt channel, through whichthe glass melt flows in a predefined direction, particularlycontinuously.

According to a preferred embodiment, the width of the marginal gapbetween the front ends of the stirrer blades and the inside surface ofthe melt receptacle or stirrer vessel is greater than approximately 5%to approximately 20%, and preferably greater than approximately 5% to amaximum of approximately 15% of the diameter of the stirring device. Asa result, the marginal gap is comparatively wide and, according to theinvention, undesirable interference, such as abrasion or corrosion ofmaterial of the wall of the melt receptacle and/or of the stirringdevice, can be avoided.

According to a further embodiment, at least one such stirring device, asdescribed above, is used for controlling a mass flow of the glass meltin the melt receptacle, regardless of the temperature and/or viscosityof the glass melt. For this purpose, the rotational stirrer speed can,in particular, be suitably controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail hereinafter by way ofexample and with reference to the accompanying drawings, which disclosefurther characteristics, advantages and objectives to be achieved,wherein:

FIG. 1 is a schematic sectional view of a device according to a firstembodiment of the present invention;

FIG. 2 a shows a conventional stirring device;

FIG. 2 b shows the configuration of the stirring device according toFIG. 2 a in a cylindrical stirrer vessel, which is basically suited forperforming the method according to the present invention;

FIG. 3 a shows a stirring device according to one exemplary embodimentof the present invention;

FIG. 3 b shows the arrangement of the stirring device according to FIG.3 a in a cylindrical stirrer vessel;

FIG. 4 a and FIG. 4 b shows a schematic sectional side view and aschematic top view of the series connection of a plurality of stirringdevices according to the present invention for forming virtual stirringdevices in a glass melt channel according to a further exemplaryembodiment of the present invention; and

FIG. 5 a and FIG. 5 b shows a schematic sectional side view and aschematic top view of the parallel arrangement of a plurality of stirrerdevices according to the present invention for forming virtual stirringdevices in a glass melt channel according to a further exemplaryembodiment of the present invention.

In the figures; identical reference numerals denote identical orsubstantially equivalent elements or groups of elements.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to FIG. 1, a stirrer, which will be described hereinafter inmore detail with reference to FIGS. 2 a and 3 a, comprising a pluralityof stirrer blades 11 is disposed in a generally cylindrical stirrervessel 2 in a point-symmetrical configuration. A glass melt 3 isreceived in the receptacle 2. The glass melt 3 can flow continuously ordiscontinuously through the stirrer vessel 2, specifically from theinlet 4 to the outlet 5. As is indicated by the arrow 12, an axial feedaction is applied in the inner stirring region, between the stirrershaft 10 and the front ends of the stirrer blades 11, which feeds theentering glass melt 3 from the upper axial end of the inner region 12 tothe lower axial end thereof. This is achieved by suitably configuringthe stirrer, which will be described in more detail hereinafter. Theglass melt exiting at the lower axial end of the inner stirring region12 brings about active sealing of the gap 16 between the front ends ofthe stirrer blades 11 and the inside wall of the cylindrical stirrervessel 2 in the region of the stirring device such that the enteringglass melt, and particularly the glass melt entering through the inlet4, cannot flow directly through the gap 16 to the lower axial end of theinner stirring region 12 and can also not directly reach the stirrershaft 10, but instead, as is indicated by the arrow, is first divertedupward and toward the upper axial end of the inner stirring region 12and is then drawn into the inner stirring region 12. In this way, themarginal gap 16 between the front ends of the stirrer blades 11 and theinside wall of the cylindrical stirrer vessel 2 is completely sealed,without requiring an excessively narrow marginal gap, which is describedin more detail hereinafter.

In this way, the striations and/or inhomogeneities in the glass melt 3are drawn into the inner stirring region 12 and stirred there, thusachieving homogenization of the glass melt.

In the example according to FIG. 1, an upward flow is produced in themarginal gap 16, the flow being indicated by the arrow, as a result ofwhich the passage of striations and/or inhomogeneities through themarginal gap 16 downward is blocked and the marginal gap is dynamicallysealed. In principle, however, such an axial upward flow is notabsolutely essential. It suffices if the marginal gap 16 is sufficientlysealed or blocked by the glass melt exiting the lower axial region ofthe inner stirring region 12 in the manner of a stopper made of glassmelt or a material accumulation.

In this way, all glass inhomogeneities, regardless of the entry pointinto the stirrer system, reach the inner stirring region between thestirrer shaft and the ends of the stirrer blades and are sufficientlyeliminated there by means of expansion, chopping and spatialredistribution. It will be apparent without difficulty to the personskilled in the art that the homogenization can be further improved bycascading two or more such stirring devices, whereby the remaining glassinhomogeneity decreases by the nth power of the remaining glassinhomogeneity, downstream of a stirring device. According to FIG. 1, theaxial feed action occurs in the direction of the general glass flow fromthe inlet 4 to the outlet 5.

It is easily apparent from FIG. 1 that sections of the cross-section ofthe inlet 4 are covered by sections of the stirrer, namely by thestirrer blades 11, such that direct entry of the inflowing glass meltinto the inner stirring region 12 is prevented. Or more precisely, inthe example according to FIG. 1 more than 50% of the cross-section ofthe inlet 4 is covered by the stirrer. Further analysis by the inventorshas shown that coverage of at least 50%, and more preferably of at leasttwo thirds, can lead to satisfactory homogenization results with acomparatively wide marginal gap 16.

FIG. 2 a shows an example of a conventional stirrer. According to FIG. 2a, the stirrer comprises a cylindrical stirrer shaft 10, from the outercircumference of which diametrically opposed pairs of cylindrical radialprotrusions 11 having faces 13 with circular profiles project axiallyoffset in relation to one another. According to FIG. 2 a, a total offive pairs of stirrer blades 11 are disposed on the outer circumferenceof the stirrer shaft 10 in an overall helical configuration. Analysis bythe inventors demonstrated that an axial feed action and sealing of themarginal gap as defined by the invention cannot be produced by the axialfeed action of such a conventional stirrer.

FIG. 2 b shows the arrangement of such a stirrer in a cylindricalstirrer vessel having an inlet 4 and an outlet 5 for the glass melt 3.If D denotes the inside diameter of the cylindrical stirrer vessel and dthe diameter of the stirrer blades 11, the following equation holds forthe marginal gap: s=(D−d)/2. This marginal gap is adjusted such that acertain level of homogenization can be achieved. It has been shown thatthe relative marginal gap s/D can be adjusted to be considerably smallerthan in a stirrer according to the invention. Typically, the relativemarginal gap s/D must be selected at considerably smaller than 5%.

The line in FIG. 2 b schematically illustrates the results of physicalsimulations of the operation of a stirring device of this type. For thispurpose, a stirring device having transparent walls was set up and thiswas operated with a transparent fluid of a comparable viscosity thatwith the intended operating conditions. A color stripe was produced withdye in the fluid flowing in through the inlet 4. Thus, it was possibleto visually observe and optically evaluate the homogenization of thefluid.

As is apparent from FIG. 2 b, the inflowing fluid is not diverted towardthe upper region of the stirrer, but instead it enters the innerstirring region of the stirrer directly from the inlet 4. As thespiral-shaped line indicates, the color stripe is swirled to a certaindegree. Only at the upper end of the outlet 5 was a relatively narrowcolor stripe observed which, as the dot density indicates, had acomparatively high dye concentration in a central region that decreasedtoward the marginal regions of the stripe extending across approximatelyone third of the cross-section of the outlet 5. Overall, thus theinhomogeneities in the fluid were not uniformly distributed across theentire cross-section of the outlet 5. Further analysis on the part ofthe inventors also showed that the position and the concentrationprofile of this stripe were not independent of the entry point of thecolor stripe in the inlet 4. The homogenization level that was achievedwas therefore not satisfactory. As the line in the stripe in outlet 5indicates, certain chaotic effects occurred in the concentration profileat the outlet 5.

FIG. 3 a illustrates a stirrer according to a further exemplaryembodiment, wherein the stirrer blades 11 are configured as obliquelyangled (inclined) plates or according to a further embodiment areconfigured substantially in a paddle shape. In the example according toFIG. 3 a, a total of six pairs of stirrer blades 11 are disposed in ahelical configuration on the outer circumference of the stirrer shaft10. To this end, the stirrer shaft 10 transitions from a bevelledshoulder 15 into a widened region at the front end of the stirrer, fromwhich the stirrer blades 11 project.

FIG. 3 b shows the stirrer according to FIG. 3 a disposed in acylindrical stirrer vessel. According to FIG. 3 b, approximately 50% ofthe cross-section of the inlet 4 is covered by the stirrer blades 11 ofthe stirrer. The lines summarize the results of a physical simulation,wherein a color stripe is introduced at the lower end of the inlet 4into a transparent fluid having a viscosity comparable to that with theintended operating conditions. As is apparent from the line, first allof the fluid that enters is diverted toward the upper end of thestirrer. There, all of the entering fluid enters the inner stirringregion of the stirrer and is fed axially downward.

In FIG. 3 b the line and dot density schematically indicates the colorconcentration profile. It is readily apparent from FIG. 3 b that stronghomogenization of the fluid is already achieved in the upper third ofthe inner stirring region, so that the color stripe that entered wascompletely and uniformly distributed. The color concentration profilewas uniform across the entire cross-section of the outlet 5. This resultwas observed regardless of the entry point of the color stripe in theinlet 4.

Further analyses and mathematical simulations conducted by the inventorsshowed that direct passage of the fluid flowing in through the inlet 4to the outlet 5 is prevented because a fluid flow flowing counter to theaxial feed action develops in the marginal gap 16, so that the flowdynamically seals the marginal gap 16. In this way, all of the inflowingfluid is diverted toward the upper end of the stirrer. The entireinflowing fluid thus reaches the inner stirring region of the stirrer,thus bringing about intensive homogenization of the fluid.

Particularly good homogenization can be achieved only if the stirrerfeed device is in agreement with the direction of the glass throughputbetween the inlet 4 and the outlet 5 and if the entry of the glass meltinto the stirring device is such that one or more stirrer blades 11prevent direct entry into the inner stirring region in the vicinity ofthe stirrer shaft 10.

In this manner, substantially regardless of the entry point, allstriations and/or glass inhomogeneities must pass through the innerstirring region, which is to say the region between the stirrer shaft 10and the front ends of the stirrer blades 11, and thereby are expanded,spatially redistributed and chopped. According to the invention, highglass homogeneity is achieved in this way, without the necessity ofkeeping the gap between the stirrer vessel and stirrer blades extremelytight, for example smaller than approximately 5 mm.

Through physical or mathematical simulations, the number of stirrerblades, the shape thereof, the azimuth angle thereof and the distancesin relation to one another as well as the installation height in thestirrer vessel can be optimized for respective stirring tasks. Therotational stirrer speed is adjusted so that the best possiblehomogenization result can be achieved, without producing undesirableside effects such as reboil or excess corrosion of the materials used.

In many cases, the use of a cylindrical stirrer vessel is technicallynot desirable for a variety of reasons, for example if high massthroughput rates are to be achieved. Hereinafter a channel stirrersystem based on the principles of the present invention is describedwith reference to FIGS. 4 a and 4 b.

According to FIG. 4 a, the glass melt flows through the channel 2,wherein the melt flows in through the inlet 4 and exits the channel 2 inthe region of the outlet 5. As is apparent from the top view accordingto FIG. 4 b, a marginal gap 16 is formed between the front ends of thestirrer blades and the lateral wall of the channel 2, the width of thegap according to the invention ranging between greater thanapproximately 5% to a maximum of approximately 15% of the diameter ofthe respective melt receptacle.

The stirrers each bring about an axial feed action, which is indicatedby the arrow 12 and described above. In this way, direct entry of theglass melt into the inner stirring region of the respective stirrer isprevented by the rotating stirrer blades 11. The glass melt flowing inthrough the inlet 4 is thus first drawn upward, then diverted to theupper axial end of the front stirrer and then drawn into the innerstirring region. In this example, the rotational speed of the stirrer isselected so that the glass melt circulates multiple times in the regionof the respective stirrer, which is indicated by the flow arrows. Eachstirrer thus forms a virtual stirrer vessel as defined by the presentinvention. Only a portion of the fed glass melt is fed to a furthervirtual stirrer vessel disposed downstream, wherein due to the rotatingstirrer blades 11, direct entry of the glass melt into the innerstirring region is accordingly prevented and due to the axial feedaction of the downstream stirrer the glass melt is first drawn upwardand then diverted toward the axial end of the downstream stirrer, wherethe melt is then drawn into the inner stirring region.

The line progression and/or the dot density in FIG. 4 a schematicallyillustrates the result of a physical simulation, as described abovebased on FIG. 2 b and FIG. 3 b. It is apparent that nearly completehomogenization of the fluid was already achieved in the upper third ofthe first stirrer.

Further mathematical simulations by the inventors demonstrated that,with a channel-shaped stirrer device of this sort, the axial mass flowbrought about by the axial feed action is always larger than athroughput flow through the channel.

Due to the axial feed action, overall virtual stirrer vessels arecreated, wherein the glass melt is fed from the top to the bottom orfrom the bottom to the top, without entering the stirrer circuit. As aresult, the distance of the stirrers from the wall of the melt channelcan be increased, without inhomogeneities passing through this gap. Byoptimizing the rotational speed of the stirrers, diameter, number ofstirrer blades, feed action of the stirrer blades, the helicalarrangement thereof on the stirrer shaft and comparable parameters aswell as through mathematical and/or physical simulations, optimizedstirring results can be achieved for the respective application.

FIG. 5 a and FIG. 5 b show a further example of an inventivechannel-shaped stirring device, wherein the stirrers are not connectedin series in the passage direction of the channel, but instead aredisposed aligned along an axis, which intersects the channel at a rightangle.

As will be apparent to the person skilled in the art without difficulty,stirring devices, as described above, can be used to control the massflow of the glass melt in the melt receptacle, regardless of thetemperature and/or viscosity of the melt. The stirrers and/or stirrervessels can be made partially or entirely of precious metal or one ormore other refractory metal. Particularly preferred is the use ofprecious metal alloys, particularly a platinum-rhodium alloy, inparticular to achieve high melting temperatures.

It will be readily apparent to the person skilled in the art that theunderlying principle of the present invention for homogenizing a glassmelt can be used for the production of display glass, particularly glasspanes for LCD, OLED or plasma displays, for the production of glassceramics, of borosilicate glass or of optical glass. Due to the dynamicsealing of the marginal gap, considerably larger gap widths can beachieved, so that according to the invention material abrasion can bereduced. This also means that, according to the present invention, thecarrying away of particles and impairment of glass quality in the stateof the art, no longer occur.

LIST OF REFERENCE NUMERALS

-   1 Stirring device-   2 Melt receptacle/stirrer vessel-   3 Melt-   4 Inlet-   5 Outlet-   10 Stirrer shaft-   11 Stirrer blade-   12 Stirring region having axial feed action-   13 End face of the stirrer blade 11-   14 Axis of rotation-   15 Step-   16 Gap/marginal gap

1. A method for homogenizing a glass melt, said method comprising thesteps of: a) providing a cylindrical stirring vessel with an outlet at alower axial end thereof, with an inlet at an upper axial end thereof,and a cylindrical inside wall extending axially from said upper axialend to said lower axial end, said inlet being provided in saidcylindrical inside wall; b) admitting said glass melt into the stirringvessel laterally through said inlet provided in said cylindrical insidewall; c) arranging at least one stirring device in the stirring vessel,each of said at least one stirring device comprising a stirrer shaft anda plurality of stirrer blades extending from said stirrer shaft; d)configuring said stirrer blades and said stirrer shaft to produce anaxial feed action in an inner stirring region between the stirrer shaftand ends of the stirrer blades facing said inside wall, in order toconvey said glass melt in said inner stirring region along the stirrershaft in a throughput direction in which the glass melt flows throughsaid stirring vessel from said inlet to said outlet; and e) configuringthe stirrer blades, the stirrer shaft, and the stirring vessel so that amelt flow in an opposite direction from that of said axial feed actionoccurs in a gap formed between said inside wall of the at least onestirring vessel and said ends of the stirrer blades, said melt flow iscaused by said axial feed action in said inner stirring region anddynamically seals said gap, whereby the glass melt entering through saidinlet in said inside wall is prevented from passing directly throughsaid gap from said inlet to said outlet; and f) providing said axialfeed action within said inner stirring region with said at least onestirring device, so that said glass melt is conveyed within said innerstirring region in said throughput direction in which the glass meltflows through said stirring vessel from the inlet to the outlet; whereinsaid configuring the stirrer blades, the stirrer shaft, and the stirringvessel so that said melt flow in said opposite direction from that ofsaid axial feed action occurs in said gap comprises arranging at leastone of said stirrer blades adjacent to said inlet so as to extend acrossa portion of a cross-section of said inlet and thus prevent direct entryof the glass melt entering the stirring vessel through the inlet intosaid inner stirring region.
 2. The method according to claim 1, whereinsaid at least one of said stirrer blades adjacent to said inlet coversat least 50% of said cross-section of said inlet provided in said insidewall.
 3. The method according to claim 1, wherein said at least one ofsaid stirrer blades adjacent to said inlet covers at least two thirds ofsaid cross-section of said inlet provided in said inside wall.
 4. Themethod according to claim 1, wherein said gap has a width greater than5% and up to 15% of an inside diameter of the stirring vessel, so thatmaterial abrasion is prevented and a bubble shear rate is low.
 5. Themethod according to claim 1, wherein said stirrer blades are obliquelyangled, arranged in a helical arrangement and/or have a geometric shape,so as to provide said axial feed action.
 6. The method according toclaim 1, wherein the glass melt flows continuously in said throughputdirection through the stirring vessel and the axial feed action occursin said throughput direction.
 7. The method according to claim 1,wherein a plurality of said stirring devices are arranged in series inthe stirring vessel.
 8. The method according to claim 7, wherein each ofthe at least one stirring devices in the stirring vessel forms a virtualvessel, with an axial mass flow caused by said axial feed action that isgreater than a throughput flow through the stirring vessel.
 9. Themethod according to claim 8, wherein the stirring vessel is configured,at least in sections thereof, as a channel through which the glass meltflows in a predefined direction.
 10. The method according to claim 1,wherein said at least one stirring device controls a mass flow of theglass melt in the stirring vessel, regardless of temperature andviscosity of the glass melt.
 11. A method for homogenizing a glass melt,said method comprising the steps of: a) providing a cylindrical stirringvessel with an outlet at a lower axial end thereof, with an inlet at anupper axial end thereof, and a cylindrical inside wall extending axiallyfrom said upper axial end to said lower axial end, said inlet beingprovided in said cylindrical inside wall; b) admitting said glass meltinto the stirring vessel laterally through said inlet provided in saidcylindrical inside wall; c) arranging at least one stirring device inthe stirring vessel, each of said at least one stirring devicecomprising a stirrer shaft and a plurality of stirrer blades extendingfrom said stirrer shaft; d) configuring said stirrer blades and saidstirrer shaft to produce an axial feed action in an inner stirringregion between the stirrer shaft and ends of the stirrer blades, inorder to convey said glass melt in said inner stirring region along thestirrer shaft in a throughput direction in which the glass melt flowsthrough said stirring vessel from said inlet to said outlet; e)configuring the stirrer blades, the stirrer shaft and the stirringvessel, so that a direct passage of the glass melt from the inlet to theoutlet through a gap formed between the inside wall of the stirringvessel and said ends of the stirrer blades facing the inside wall iseffectively prevented by a melt flow in a direction opposite to that ofsaid axial feed action and so that said gap has a width greater than 5%and up to 15% of an inside diameter of the stirring vessel in order toprevent material abrasion and provide a low bubble shear rate; f)configuring the stirrer blades, the stirrer shaft, and the stirringvessel so that at least one of said stirrer blades is arranged adjacentto said inlet and extends across a portion of a cross-section of saidinlet thereby preventing direct entry of the glass melt entering thestirring vessel through the inlet into said inner stirring region; andg) providing said axial feed action within said inner stirring regionwith said at least one stirring device, so that said glass melt isconveyed within said inner stirring region in said throughput directionin which the glass melt flows through said stirring vessel from theinlet to the outlet.
 12. The method according to claim 11, wherein saidat least one of said stirrer blades adjacent said inlet covers at least50% of a cross-section of said inlet provided in said inside wall. 13.The method according to claim 11, wherein said at least one of saidstirrer blades adjacent said inlet covers at least two thirds of across-section of said inlet provided in said inside wall.
 14. The methodaccording to claim 11, wherein said stirrer blades are obliquely angled,arranged in a helical arrangement and/or have a geometric shape, so asto provide said axial feed action.
 15. The method according to claim 11,wherein the glass melt flows continuously in said throughput directionthrough the stirring vessel and the axial feed action occurs in saidthroughput direction.
 16. The method according to claim 11, wherein aplurality of said stirring devices are arranged in series in thestirring vessel.
 17. The method according to claim 16, wherein each ofsaid stirring devices in the stirring vessel forms a virtual vessel,with an axial mass flow caused by said axial feed action that is greaterthan a throughput flow through the stirring vessel.
 18. The methodaccording to claim 17, wherein the stirring vessel is configured, atleast in sections thereof, as a channel through which the glass meltflows in a predefined direction.
 19. The method according to claim 11,wherein said at least one stirring device controls a mass flow of theglass melt in the stirring vessel, regardless of temperature andviscosity of the glass melt.